Function in_flight_saturated 

pub fn in_flight_saturated(in_flight: usize, max_in_flight: usize) -> bool

Whether the in-flight set is saturated and the ventilator should apply backpressure (stop leasing new work and mock-reply). Saturation is inclusive: at the threshold we already hold back, keeping the set bounded below PROGRESS_QUEUE_HARD_LIMIT.